Method for transmitting harq ack/nack and wireless device using same

ABSTRACT

Provided are a method for transmitting hybrid automatic repeat request (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) in a wireless communication system and a wireless device using the same. The wireless device receives downlink control information from a first base station on a downlink control channel and receives a downlink transmission block on a downlink shared channel according to the downlink control information received from the first base station. The wireless device transmits ACK/NACK to a second base station in response to the downlink transmission block on an uplink control channel. The downlink control information includes an indicator used to determine a radio resource for the uplink control channel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method of transmitting positive-acknowledgement(ACK)/negative-acknowledgement (NACK) for hybrid automatic repeatrequest (HARQ) in a wireless communication system, and a wireless deviceusing the method.

2. Related Art

Long term evolution (LTE) based on 3^(rd) generation partnership project(3GPP) technical specification (TS) release 8 is a promisingnext-generation mobile communication standard. Recently, LTE-advanced(LTE-A) based on 3GPP TS release 10 supporting multiple carriers isunder standardization.

As disclosed in 3GPP TS 36.211 V10.2.0 (2011-06) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 10)”, a physical channel of 3GPP LTE/LTE-A can be classifiedinto a downlink channel, i.e., a physical downlink shared channel(PDSCH) and a physical downlink control channel (PDCCH), and an uplinkchannel, i.e., a physical uplink shared channel (PUSCH) and a physicaluplink control channel (PUCCH).

Recently, an enhanced PDCCH (EPDCCH) for more flexible scheduling of acontrol channel has been introduced. The EPDCCH supports various schemessuch as a search space in a PDSCH region, multiple antenna transmission,etc.

For an HARQ operation, a PDCCH-PUCCH resource linkage is established in3GPP LTE/LTE-A. Upon receiving the PDCCH, a user equipment sends HARQACK/NACK through a PUCCH corresponding to the PDCCH by using a resourcelinkage. This is to implicitly exchange information about to whichtransport block the HARQ ACK/NACK belongs between a base station and theuser equipment.

The EPDCCH supports a coordinated multi-point (CoMP). In the CoMPoperation, the UE receives the EPDCCH from a first base station, and maytransmit a corresponding PUCCH to a second base station. That is, thisis a case where a downlink base station and an uplink base station aredifferent from each other. Since the base stations are different, thereis a need to modify the PDCCH-PUCCH resource linkage.

SUMMARY OF THE INVENTION

The present invention provides a method of transmittingpositive-acknowledgement (ACK)/negative-acknowledgement (NACK) forhybrid automatic repeat request (HARQ) in a wireless communicationsystem, and a wireless device using the method.

In an aspect, a method for transmitting hybrid automatic repeat request(HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) ina wireless communication system is provided. The method includesreceiving, by a wireless device, downlink control information from afirst base station on a downlink control channel, receiving, by thewireless device, a downlink transport block from the first base stationon a downlink shared channel according to the downlink controlinformation, and transmitting, by the wireless device, ACK/NACK for thedownlink transport block to a second base station on an uplink controlchannel. Information regarding a cell identity for the uplink controlchannel for the second base station is received from the first basestation, and the downlink control information includes an indicator usedto determine a radio resource for the uplink control channel.

The method may further include receiving, by the wireless device,information regarding a plurality of uplink channel resource candidatesfrom the first base station. The indicator may indicate one of theplurality of uplink channel resource candidates.

The transmitting of the ACK/NACK on the uplink control channel mayinclude modulating the ACK/NACK to generate a modulation symbol,spreading the modulation symbol to a sequence which is cyclicallyshifted by a cyclic shift amount, and transmitting the spread sequence.

The cyclic shift amount may be determined based on the cell identity.

In another aspect, a wireless device for transmitting hybrid automaticrepeat request (HARQ) positive-acknowledgement(ACK)/negative-acknowledgement (NACK) in a wireless communication systemis provided. The wireless device includes a radio frequency (RF) unitconfigured to transmit and receive a radio signal, and a processoroperatively coupled to the RF unit and configured to receive downlinkcontrol information from a first base station on a downlink controlchannel, receive a downlink transport block from the first base stationon a downlink shared channel according to the downlink controlinformation, and transmit ACK/NACK for the downlink transport block to asecond base station on an uplink control channel. Information regardinga cell identity for the uplink control channel for the second basestation is received from the first base station, and the downlinkcontrol information includes an indicator used to determine a radioresource for the uplink control channel.

A radio resource for an uplink control channel which is linked to aresource of an enhanced physical downlink control channel (EPDCCH) canbe ensured. The radio resource for the uplink control channel can beensured in various coordinated multi-point (CoMP) environments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a downlink (DL) radio frame in 3^(rd)generation partnership project (3GPP) long term evolution (LTE).

FIG. 2 shows a structure of an uplink (UL) subframe in 3GPP LTE.

FIG. 3 shows a diagram showing a configuration of a physical downlinkcontrol channel (PDCCH).

FIG. 4 shows an example of monitoring a PDCCH.

FIG. 5 shows an example of arranging a reference signal and a controlchannel in a DL subframe of 3GPP LTE.

FIG. 6 shows an example of a subframe having an enhanced PDCCH (EPDCCH).

FIG. 7 shows an example of a physical resource block (PRB) pair.

FIG. 8 shows a DL hybrid automatic repeat request (HARQ) operation in3GPP LTE.

FIG. 9 shows transmission of a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal when a physical uplinkcontrol channel (PUCCH) format 1b is used in a normal cyclic prefix (CP)case in 3GPP LTE.

FIG. 10 shows a structure of a PUCCH format 3 in a normal CP case.

FIG. 11 shows an example of a CoMP scenario.

FIG. 12 shows a PUCCH resource allocation according to an embodiment ofthe present invention.

FIG. 13 shows a PUCCH resource allocation according to anotherembodiment of the present invention.

FIG. 14 is another example of a CoMP scenario.

FIG. 15 shows a PUCCH resource allocation according to anotherembodiment of the present invention.

FIG. 16 is a flowchart showing an ACK/NACK transmission method accordingto an embodiment of the present invention.

FIG. 17 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A wireless device may be fixed or mobile, and may be referred to asanother terminology, such as a user equipment (UE), a mobile station(MS), a mobile terminal (MT), a user terminal (UT), a subscriber station(SS), a personal digital assistant (PDA), a wireless modem, a handhelddevice, etc. The wireless device may also be a device supporting onlydata communication such as a machine-type communication (MTC) device.

A base station (BS) is generally a fixed station that communicates withthe wireless device, and may be referred to as another terminology, suchas an evolved-NodeB (eNB), a base transceiver system (BTS), an accesspoint, etc.

Hereinafter, it is described that the present invention is appliedaccording to a 3rd generation partnership project (3GPP) long termevolution (LTE) based on 3GPP technical specification (TS) release 8 or3GPP LTE-advanced (LTE-A) based on 3GPP TS release 10. However, this isfor exemplary purposes only, and thus the present invention is alsoapplicable to various wireless communication networks. In the followingdescription, LTE and/or LTE-A are collectively referred to as LTE.

The wireless device may be served by a plurality of serving cells. Eachserving cell may be defined with a downlink (DL) component carrier (CC)or a pair of a DL CC and an uplink (UL) CC.

The serving cell may be classified into a primary cell and a secondarycell. The primary cell operates at a primary frequency, and is a celldesignated as the primary cell when an initial network entry process isperformed or when a network re-entry process starts or in a handoverprocess. The primary cell is also called a reference cell. The secondarycell operates at a secondary frequency. The secondary cell may beconfigured after an RRC connection is established, and may be used toprovide an additional radio resource. At least one primary cell isconfigured always. The secondary cell may be added/modified/released byusing higher-layer signaling (e.g., a radio resource control (RRC)message).

A cell index (CI) of the primary cell may be fixed. For example, alowest CI may be designated as the CI of the primary cell. It is assumedhereinafter that the CI of the primary cell is 0 and a CI of thesecondary cell is allocated sequentially starting from 1.

FIG. 1 shows a structure of a DL radio frame in 3GPP LTE-A. The section6 of 3GPP TS 36.211 V10.2.0 (2011-06) “Evolved Universal TerrestrialRadio Access (E-UTRA); Physical Channels and Modulation (Release 10)”may be incorporated herein by reference.

A radio frame includes 10 subframes indexed with 0 to 9. One subframeincludes 2 consecutive slots. A time required for transmitting onesubframe is defined as a transmission time interval (TTI). For example,one subframe may have a length of 1 millisecond (ms), and one slot mayhave a length of 0.5 ms.

One slot may include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlink(DL), the OFDM symbol is only for expressing one symbol period in thetime domain, and there is no limitation in multiple access schemes orterminologies. For example, the OFDM symbol may also be referred to asanother terminology such as a single carrier frequency division multipleaccess (SC-FDMA) symbol, a symbol period, etc.

Although it is described that one slot includes 7 OFDM symbols forexample, the number of OFDM symbols included in one slot may varydepending on a length of a cyclic prefix (CP). According to 3GPP TS36.211 V10.2.0, in case of a normal CP, one slot includes 7 OFDMsymbols, and in case of an extended CP, one slot includes 6 OFDMsymbols.

A resource block (RB) is a resource allocation unit, and includes aplurality of subcarriers in one slot. For example, if one slot includes7 OFDM symbols in a time domain and the RB includes 12 subcarriers in afrequency domain, one RB can include 7×12 resource elements (REs).

A DL subframe is divided into a control region and a data region in thetime domain. The control region includes up to first four OFDM symbolsof a first slot in the subframe. However, the number of OFDM symbolsincluded in the control region may vary. A physical downlink controlchannel (PDCCH) and other control channels are allocated to the controlregion, and a physical downlink shared channel (PDSCH) is allocated tothe data region.

As disclosed in 3GPP TS 36.211 V10.2.0, examples of a physical controlchannel in 3GPP LTE/LTE-A include a physical downlink control channel(PDCCH), a physical control format indicator channel (PCFICH), and aphysical hybrid-ARQ indicator channel (PHICH).

The PCFICH transmitted in a first OFDM symbol of the subframe carries acontrol format indicator (CFI) regarding the number of OFDM symbols(i.e., a size of the control region) used for transmission of controlchannels in the subframe. A wireless device first receives the CFI onthe PCFICH, and thereafter monitors the PDCCH.

Unlike the PDCCH, the PCFICH does not use blind decoding, and istransmitted by using a fixed PCFICH resource of the subframe.

The PHICH carries a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal for an uplink hybridautomatic repeat request (HARQ). The ACK/NACK signal for uplink (UL)data on a PUSCH transmitted by the wireless device is transmitted on thePHICH.

A physical broadcast channel (PBCH) is transmitted in first four OFDMsymbols in a second slot of a first subframe of a radio frame. The PBCHcarries system information necessary for communication between thewireless device and a BS. The system information transmitted through thePBCH is referred to as a master information block (MIB). In comparisonthereto, system information transmitted on the PDCCH is referred to as asystem information block (SIB).

FIG. 2 shows a structure of a UL subframe in 3GPP LTE.

A UL subframe can be divided into a control region and a data region infrequency domain. The control region is a region to which a physicaluplink control channel (PUCCH) is allocated. The data region is a regionto which a physical uplink shared channel (PUSCH) is allocated.

The PUCCH is allocated in an RB pair in a subframe. RBs belonging to theRB pair occupy different subcarriers in each of a 1st slot and a 2ndslot. m is a location index indicating a logical frequency-domainlocation of the RB pair allocated to the PUCCH in the subframe.

It shows that RBs having the same value m occupy different subcarriersin the two slots.

The PUSCH is allocated by a UL grant on a PDCCH. Although not shown, a4th OFDM symbol of each slot of a normal CP is used in transmission of ademodulation reference signal (DM RS).

Uplink control information (UCI) includes at least any one of HARQACK/NACK, channel state information (CSI), and scheduling request (SR).Hereinafter, as an index of indicating a state of a DL channel, the CSImay include at least any one of a channel quality indicator (CQI) and aprecoding matrix indicator (PMI).

PUCCH formats are defined as a combination of UCI and PUCCH in order totransmit various UCI on PUCCH.

TABLE 1 PUCCH format Transmitted UCI PUCCH format 1 Positive SR PUCCHformat 1a/1b 1 bit or 2 bits HARQ ACCK/NACK PUCCH format 2 CSI reportPUCCH format 2a/2b CSI report and 1 bit or 2 bits HARQ ACCK/NACK PUCCHformat 3 HARQ ACCK/NACK, SR, CSI

A PUCCH format 1a/1b is used to carry 1 bit or 2 bits HARQ ACK/NACK byusing binary phase shift keying (BPSK) modulation or quadrature phaseshift keying (QPSK) modulation.

A PUCCH format 3 can be used to transmit 48 bits encoded UCI. The PUCCHformat 3 can carry HARQ ACK/NACK for a plurality of serving cells and aCSI report for a single serving cell.

Hereinafter, transmissions of PDCCH and reference signals are described.

Control information transmitted through the PDCCH is referred to asdownlink control information (DCI). The DCI may include resourceallocation of the PDSCH (this is referred to as a downlink (DL) grant),resource allocation of a PUSCH (this is referred to as an uplink (UL)grant), a set of transmit power control commands for individual UEs inany UE group, and/or activation of a voice over Internet protocol(VoIP).

In 3GPP LTE/LTE-A, transmission of a DL transport block is performed ina pair of the PDCCH and the PDSCH. Transmission of a UL transport blockis performed in a pair of the PDCCH and the PUSCH. For example, thewireless device receives the DL transport block on a PDSCH indicated bythe PDCCH. The wireless device receives a DL resource assignment on thePDCCH by monitoring the PDCCH in a DL subframe. The wireless devicereceives the DL transport block on a PDSCH indicated by the DL resourceassignment.

FIG. 3 is a block diagram showing a structure of a PDCCH.

The 3GPP LTE/LTE-A uses blind decoding for PDCCH detection. The blinddecoding is a scheme in which a desired identifier is de-masked from acyclic redundancy check (CRC) of a received PDCCH (referred to as acandidate PDCCH) to determine whether the PDCCH is its own controlchannel by performing CRC error checking.

A BS determines a PDCCH format according to DCI to be transmitted to awireless device, attaches a CRC to control information, and masks aunique identifier (referred to as a radio network temporary identifier(RNTI)) to the CRC according to an owner or usage of the PDCCH (block210).

If the PDCCH is for a specific wireless device, a unique identifier(e.g., cell-RNTI (C-RNTI)) of the wireless device may be masked to theCRC. Alternatively, if the PDCCH is for a paging message, a pagingindication identifier (e.g., paging-RNTI (P-RNTI)) may be masked to theCRC. If the PDCCH is for system information, a system informationidentifier (e.g., system information-RNTI (SI-RNTI)) may be masked tothe CRC. To indicate a random access response that is a response fortransmission of a random access preamble of the wireless device, arandom access-RNTI (RA-RNTI) may be masked to the CRC. To indicate atransmit power control (TPC) command for a plurality of wirelessdevices, a TPC-RNTI may be masked to the CRC.

When the C-RNTI is used, the PDCCH carries control information for aspecific wireless device (such information is called UE-specific controlinformation), and when other RNTIs are used, the PDCCH carries commoncontrol information received by all or a plurality of wireless devicesin a cell.

The CRC-attached DCI is encoded to generate coded data (block 220).Encoding includes channel encoding and rate matching.

The coded data is modulated to generate modulation symbols (block 230).

The modulation symbols are mapped to physical resource elements (REs)(block 240). The modulation symbols are respectively mapped to the REs.

A control region in a subframe includes a plurality of control channelelements (CCEs). The CCE is a logical allocation unit used to providethe PDCCH with a coding rate depending on a radio channel state, andcorresponds to a plurality of resource element groups (REGs). The REGincludes a plurality of REs. According to an association relation of thenumber of CCEs and the coding rate provided by the CCEs, a PDCCH formatand a possible number of bits of the PDCCH are determined.

One REG includes 4 REs. One CCE includes 9 REGs. The number of CCEs usedto configure one PDCCH may be selected from a set {1, 2, 4, 8}. Eachelement of the set {1, 2, 4, 8} is referred to as a CCE aggregationlevel.

The BS determines the number of CCEs used in transmission of the PDCCHaccording to a channel state. For example, a wireless device having agood DL channel state can use one CCE in PDCCH transmission. A wirelessdevice having a poor DL channel state can use 8 CCEs in PDCCHtransmission.

A control channel consisting of one or more CCEs performs interleavingon an REG basis, and is mapped to a physical resource after performingcyclic shift based on a cell identifier (ID).

FIG. 4 shows an example of monitoring a PDCCH. The section 9 of 3GPP TS36.213 V10.2.0 (2011-06) can be incorporated herein by reference.

The 3GPP LTE uses blind decoding for PDCCH detection. The blind decodingis a scheme in which a desired identifier is de-masked from a CRC of areceived PDCCH (referred to as a candidate PDCCH) to determine whetherthe PDCCH is its own control channel by performing CRC error checking. Awireless device cannot know about a specific position in a controlregion in which its PDCCH is transmitted and about a specific CCEaggregation or DCI format used for PDCCH transmission.

A plurality of PDCCHs can be transmitted in one subframe. The wirelessdevice monitors the plurality of PDCCHs in every subframe. Monitoring isan operation of attempting PDCCH decoding by the wireless deviceaccording to a format of the monitored PDCCH.

The 3GPP LTE uses a search space to reduce a load of blind decoding. Thesearch space can also be called a monitoring set of a CCE for the PDCCH.The wireless device monitors the PDCCH in the search space.

The search space is classified into a common search space and aUE-specific search space. The common search space is a space forsearching for a PDCCH having common control information and consists of16 CCEs indexed with 0 to 15. The common search space supports a PDCCHhaving a CCE aggregation level of {4, 8}. However, a PDCCH (e.g., DCIformats 0, 1A) for carrying UE-specific information can also betransmitted in the common search space. The UE-specific search spacesupports a PDCCH having a CCE aggregation level of {1, 2, 4, 8}.

Table 2 shows the number of PDCCH candidates monitored by the wirelessdevice.

TABLE 1 Search Aggregation Size Number of PDCCH DCI Space Type level L[in CCEs] candidates formats UE- 1 6 6 0, 1, 1A, specific 2 12 6 1B, 1D,4 8 2 2, 2A 8 16 2 Common 4 16 4 0, 1A, 1C, 8 16 2 3/3A

A size of the search space is determined by Table 2 above, and a startpoint of the search space is defined differently in the common searchspace and the UE-specific search space. Although a start point of thecommon search space is fixed irrespective of a subframe, a start pointof the UE-specific search space may vary in every subframe according toa UE identifier (e.g., C-RNTI), a CCE aggregation level, and/or a slotnumber in a radio frame. If the start point of the UE-specific searchspace exists in the common search space, the UE-specific search spaceand the common search space may overlap with each other.

In a CCE aggregation level Lε{1,2,3,4}, a search space S^((L)) _(k) isdefined as a set of PDCCH candidates. A CCE corresponding to a PDCCHcandidate m of the search space S^((L)) _(k) is given by Equation 1below.

L·{(Y _(k) +m′)mod └N _(CCE,k) /L┘}+i  [Equation 1]

Herein, i=0,1, . . . ,L−1, m=0, . . . ,M^((L))−1, and N_(CCE,k) denotesthe total number of CCEs that can be used for PDCCH transmission in acontrol region of a subframe k. The control region includes a set ofCCEs numbered from 0 to N_(CCE,k-1). M^((L)) denotes the number of PDCCHcandidates in a CCE aggregation level L of a given search space.

If a carrier indicator field (CIF) is set to the wireless device,m′=m+M^((L))n_(cif). Herein, n_(cif) is a value of the CIF. If the CIFis not set to the wireless device, m′=m.

In a common search space, Y_(k) is set to 0 with respect to twoaggregation levels L=4 and L=8.

In a UE-specific search space of the aggregation level L, a variableY_(k) is defined by Equation 2 below.

Y _(k)=(A·Y _(k-1))mod D  [Equation 2]

Herein, Y⁻¹=n_(RNTI)≠0, A=39827, D=65537, k=floor(n_(s)/2), and n_(s)denotes a slot number in a radio frame.

When the wireless device monitors the PDCCH by using the C-RNTI, asearch space and a DCI format used in monitoring are determinedaccording to a transmission mode of the PDSCH. Table 3 below shows anexample of PDCCH monitoring in which the C-RNTI is set.

TABLE 2 Transmis- Transmission mode of sion mode DCI format search spacePDSCH based on PDCCH Mode 1 DCI format 1A common Single-antenna port,port 0 and UE specific DCI format 1 UE specific Single-antenna port,port 0 Mode 2 DCI format 1A common Transmit diversity and UE specificDCI format 1 UE specific Transmit diversity Mode 3 DCI format 1A commonTransmit diversity and UE specific DCI format 2A UE specific CDD(CyclicDelay Diversity) or Transmit diversity Mode 4 DCI format 1A commonTransmit diversity and UE specific DCI format 2 UE-specific Closed-loopspatial multi- plexing Mode 5 DCI format 1A common Transmit diversityand UE specific DCI format 1D UE specific MU-MIMO(Multi-User MultipleInput Multiple Output) Mode 6 DCI format 1A common Transmit diversityand UE specific DCI format 1B UE specific Closed-loop spatial multi-plexing Mode 7 DCI format 1A common If the number of PBCH and UEtransmission ports is 1, specific single antenna port, port 0, and ifnot, Transmit diversity DCI format 1 UE specific Single antenna port,port 5 Mode 8 DCI format 1A common If the number of PBCH and UEtransmission ports is 1, specific single antenna port, port 0, and ifnot, Transmit diversity DCI format 2B UE specific Dual layertransmission (port 7 or 8), or a single antenna port, port 7 or 8

FIG. 5 shows an example of arranging a reference signal and a controlchannel in a DL subframe of 3GPP LTE.

A control region (or a PDCCH region) includes first three OFDM symbols,and a data region in which a PDSCH is transmitted includes the remainingOFDM symbols.

A PCFICH, a PHICH, and/or a PDCCH are transmitted in the control region.A control format indictor (CFI) of the PCFICH indicates three OFDMsymbols. A region excluding a resource in which the PCFICH and/or thePHICH are transmitted in the control region is a PDCCH region whichmonitors the PDCCH.

Various reference signals are transmitted in the subframe.

A cell-specific reference signal (CRS) may be received by all wirelessdevices in a cell, and is transmitted across a full downlink frequencyband. In FIG. 4, ‘R0’ indicates a resource element (RE) used to transmita CRS for a first antenna port, ‘R1’ indicates an RE used to transmit aCRS for a second antenna port, ‘R2’ indicates an RE used to transmit aCRS for a third antenna port, and ‘R3’ indicates an RE used to transmita CRS for a fourth antenna port.

An RS sequence r_(1,ns)(m) for a CRS is defined as follows.

$\begin{matrix}{{r_{l,{n\; s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Herein, m=0,1, . . . ,2N_(maxRB)−1. N_(maxRB) is the maximum number ofRBs. ns is a slot number in a radio frame. l is an OFDM symbol index ina slot.

A pseudo-random sequence c(i) is defined by a length-31 gold sequence asfollows.

c(n)=(x ₁(n+Nc)+x ₂(n+Nc)mod 2

x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2

x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₂(n+1)+x ₂(n))mod 2  [Equation 4]

Herein, Nc=1600, and a first m-sequence is initialized as x₁(0)=1,x₁(n)=0, m=1,2, . . . ,30.

A second m-sequence is initialized asc_(init)=2¹⁰(7(ns+1)+1+1)(2N^(cell) _(ID)+1)+2 N^(cell) _(ID)+N_(CP) ata start of each OFDM symbol. N^(cell) _(ID) is a physical cellidentifier (PCI). N_(cp)=1 in a normal CP case, and N_(CP)=0 in anextended CP case.

A UE-specific reference signal (URS) is transmitted in the subframe.Whereas the CRS is transmitted in the entire region of the subframe, theURS is transmitted in a data region of the subframe and is used todemodulate the PDSCH. In FIG. 4, ‘R5’ indicates an RE used to transmitthe URS. The URS is also called a dedicated reference signal (DRS) or ademodulation reference signal (DM-RS).

The URS is transmitted only in an RB to which a corresponding PDSCH ismapped. Although R5 is indicated in FIG. 4 in addition to a region inwhich the PDSCH is transmitted, this is for indicating a location of anRE to which the URS is mapped.

The URS is used only by a wireless device which receives a correspondingPDSCH. A reference signal (RS) sequence r_(ns)(m) for the URS isequivalent to Equation 3. In this case, m=0,1, . . . ,12N_(PDSCH,RB)−1,and N_(PDSCH,RB) is the number of RBs used for transmission of acorresponding PDSCH. A pseudo-random sequence generator is initializedas c_(init)=(floor(ns/2)+1)(2 N^(cell) _(ID))2¹⁶+n_(RNTI) at a start ofeach subframe. n_(RNTI) is an identifier of the wireless device.

The aforementioned initialization method is for a case where the URS istransmitted through the single antenna, and when the URS is transmittedthrough multiple antennas, the pseudo-random sequence generator isinitialized as c_(init)=(floor(ns/2)+1)(2 N^(cell) _(ID))2¹⁶+n_(SCID) ata start of each subframe. n_(SCID) is a parameter acquired from a DLgrant (e.g., a DCI format 2B or 2C) related to PDSCH transmission.

Meanwhile, the PDCCH is monitored in an area restricted to the controlregion in the subframe, and a CRS transmitted in a full band is used todemodulate the PDCCH. As a type of control data is diversified and anamount of control data is increased, scheduling flexibility is decreasedwhen using only the existing PDCCH. In addition, in order to decrease anoverhead caused by CRS transmission, an enhanced PDCCH (EPDCCH) isintroduced.

FIG. 6 is an example of a subframe having an EPDCCH.

The subframe may include zero or one PDCCH region 410 and zero or moreEPDCCH regions 420 and 430.

The EPDCCH regions 420 and 430 are regions in which a wireless devicemonitors the EPDCCH. The PDCCH region 410 is located in up to first fourOFDM symbols of the subframe, whereas the EPDCCH regions 420 and 430 maybe flexibly scheduled in an OFDM symbol located after the PDCCH region410.

One or more EPDCCH regions 420 and 430 may be assigned to the wirelessdevice. The wireless device may monitor EPDDCH data in the assignedEPDCCH regions 420 and 430.

The number/location/size of the EPDCCH regions 420 and 430 and/orinformation regarding a subframe for monitoring the EPDCCH may bereported by a BS to the wireless device by using a radio resourcecontrol (RRC) message or the like.

In the PDCCH region 410, a PDCCH may be demodulated on the basis of aCRS. In the EPDCCH regions 420 and 430, instead of the CRS, a DM-RS maybe defined for demodulation of the EPDCCH. An associated DM-RS may betransmitted in the EPDCCH regions 420 and 430.

An RS sequence for the associated DM-RS is equivalent to Equation 3. Inthis case, m=0, 1, 12N_(RB)−1, and N_(RB) is a maximum number of RBs. Apseudo-random sequence generator may be initialized asc_(init)=(floor(ns/2)+1)(2 N_(EPDCCH,ID)+1)2¹⁶+n_(EPDCCH,SCID) at astart of each subframe. ns is a slot number of a radio frame.N_(EPDCCH,ID) is a cell index related to a corresponding EPDCCH region.n_(EPDCCH,SCID) is a parameter given from higher layer signaling.

Each of the EPDCCH regions 420 and 430 may be used to schedule adifferent cell. For example, an EPDCCH in the EPDCCH region 420 maycarry scheduling information for a primary cell, and an EPDCCH in theEPDCCH region 430 may carry scheduling information for a secondary cell.

When the EPDCCH is transmitted through multiple antennas in the EPDCCHregions 420 and 430, the same precoding as that used in the EPDCCH maybe applied to a DM-RS in the EPDCCH regions 420 and 430.

Comparing with a case where the PDCCH uses a CCE as a transmissionresource unit, a transmission resource unit for the EPDCCH is called anenhanced control channel element (ECCE). An aggregation level may bedefined as a resource unit for monitoring the EPDCCH. For example, when1 ECCE is a minimum resource for the EPDCCH, it may be defined as anaggregation level L={1, 2, 4, 8, 16}.

An EPDCCH search space may corresponds to an EPDCCH region. One or moreEPDCCH candidates may be monitored at one or more aggregation levels inthe EPDCCH search space.

Now, resource allocation for an EPDCCH will be described.

The EPDCCH is transmitted by using one or more ECCEs. The ECCE includesa plurality of enhanced resource element groups (EREGs). According to aCP and a subframe type based on a time division duplex (TDD) DL-ULconfiguration, the ECCE may include 4 EREGs or 8 EREGs. For example, theECCE may include 4 EREGs in a normal CP case, and may include 8 EREGs inan extended CP case.

A physical resource block (PRB) pair is 2 PRBs having the same RB numberin one subframe. The PRB pair is a first PRB of a first slot and asecond PRB of a second slot in the same frequency domain. In the normalCP case, the PRB pair includes 12 subcarriers and 14 OFDM symbols, andthus includes 168 resource elements (REs).

FIG. 7 shows an example of a PRB pair. Although it is assumedhereinafter that a subframe includes 2 slots and a PRB pair in one slotincludes 7 OFDM symbols and 12 subcarriers, the number of OFDM symbolsand the number of subcarriers are for exemplary purposes only.

In one subframe, the PRB pair includes 168 REs in total. 16 EREGs areconfigured from 144 REs, except for 24 REs for a DM RS. Therefore, 1EREG may include 9 REs. However, a CRS-RS or a CRS may be placed to onePRB pair, in addition to the DM RS. In this case, the number ofavailable REs may be decreased, and the number of REs included in 1 EREGmay be decreased. The number of REs included in the EREG may be changed,whereas there is no change in the number (i.e., 16) of EREGs included inone PRB pair.

In this case, as shown in FIG. 7, an RE index may be assignedsequentially starting from a first subcarrier of a first OFDM symbol(1=0). Assume that 16 EREGs are indexed from 0 to 15. In this case, 9REs having an RE index 0 are assigned to an EREG 0. Likewise, 9 REscorresponding to an RE index k (k=0, . . . , 15) are assigned to an EREGk.

An EREG group is defined by aggregating a plurality of EREGs. Forexample, if an EREG group having 4 EREGs is defined, it may be definedas an EREG group #0={EREG 0, EREG 4, EREG 8, EREG 12}, an EREG group#1={EREG 1, EREG 5, EREG 9, EREG 3}, an EREG group #2={EREG 2, EREG 6,EREG 10, EREG 14}, and an EREG group #3={EREG 3, EREG 7, EREG 11, EREG15}. If an EREG group having 8 EREGs is defined, it may be defined as anEREG group #0={EREG 0, EREG 2, EREG 4, EREG 6, EREG 8, EREG 10, EREG 12,EREG 14} and an EREG group #1={EREG 1, EREG 3, EREG 5, EREG 7, EREG 9,EREG 11, EREG 13, EREG 15}.

As described above, the ECCE may include 4 EREGs. In an extended CPcase, the ECCE may include 8 EREGs. The ECCE is defined by the EREGgroup. For example, it is exemplified in FIG. 6 that an ECCE #0 includesan EREG group #0, an ECCE #1 includes an EREG group #1, an ECCE #2includes an EREG group #2, and an ECCE #3 includes an EREG group #3.

ECCE-to-EREG mapping has two types of transmission, i.e., localizedtransmission and distributed transmission. In the localizedtransmission, an EREG group constituting one ECCE is selected from EREGsof one PRB pair. In the distributed transmission, an EREG constitutingone ECCE is selected from EREGs of different PRB pairs.

Now, a DL HARQ operation and a PUCCH structure will be described.

FIG. 8 shows a DL HARQ operation in 3GPP LTE.

A wireless device monitors a PDCCH, and receives a DL grant including aDL resource allocation on a PDCCH 501 (or EPDDCH) in an n^(th) DLsubframe. The wireless device receives a DL transport block through aPDSCH 502 indicated by the DL resource allocation.

The wireless device transmits an ACK/NACK signal for the DL transportblock on a PUCCH 511 in an (n+4)^(th) UL subframe. The ACK/NACK signalcorresponds to an ACK signal when the DL transport block is successfullydecoded, and corresponds to a NACK signal when the DL transport blockfails in decoding. Upon receiving the NACK signal, a BS may retransmitthe DL transport block until the ACK signal is received or until thenumber of retransmission attempts reaches its maximum number.

In 3GPP LTE, PUCCH formats 1 a/1b/3 are used to carry an ACK/NACK signalwhich is a reception acknowledgement for HARQ.

All PUCCH formats use a cyclic shift (CS) of a sequence in each OFDMsymbol. The cyclically shifted sequence is generated by cyclicallyshifting a base sequence by a specific CS amount. The specific CS amountis indicated by a CS index.

An example of a base sequence r_(u)(n) is defined by the followingequation.

r _(u)(n)=e ^(jb(n)π/4)  [Equation 5]

Herein, u denotes a root index, and n denotes a component index in therange of 0≦n≦N−1, where N is a length of the base sequence. b(n) isdefined in the section 5.5 of 3GPP TS 36.211 V10.2.0.

A length of a sequence is equal to the number of elements included inthe sequence. u can be determined by a cell identifier (ID), a slotnumber in a radio frame, etc. When it is assumed that the base sequenceis mapped to one resource block in a frequency domain, the length N ofthe base sequence is 12 since one resource block includes 12subcarriers. A different base sequence is defined according to adifferent root index.

The base sequence r(n) can be cyclically shifted by the followingequation to generate a cyclically shifted sequence r(n, I_(cs)).

$\begin{matrix}{{{r\left( {n,I_{cs}} \right)} = {{r(n)} \cdot {\exp \left( \frac{j\; 2\; \pi \; I_{cs}n}{N} \right)}}},{0 \leq I_{cs} \leq {N - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

Herein, I_(cs) denotes a CS index indicating a CS amount (0≦I_(cs)≦N−1).

Hereinafter, the available CS of the base sequence denotes a CS indexthat can be derived from the base sequence according to a CS interval.For example, if the base sequence has a length of 12 and the CS intervalis 1, the total number of available CS indices of the base sequence is12. Alternatively, if the base sequence has a length of 12 and the CSinterval is 2, the total number of available CS indices of the basesequence is 6.

FIG. 9 shows transmission of an ACK/NACK signal when a PUCCH format 1bis used in a normal CP case in 3GPP LTE.

One slot includes 7 OFDM symbols. Three OFDM symbols are referencesignal (RS) OFDM symbols for an RS. Four OFDM symbols are data OFDMsymbols for an ACK/NACK signal.

In the PUCCH format 1b, a modulation symbol d(0) is generated bymodulating a 2-bit ACK/NACK signal based on quadrature phase shiftkeying (QPSK).

A CS index I_(cs) may vary depending on a slot number n_(s) in a radioframe and/or a symbol index 1 in a slot.

In the normal CP case, there are four data OFDM symbols for transmissionof the ACK/NACK signal in one slot. Therefore, assume that CS indicescorresponding to the respective data OFDM symbols are denoted byI_(cs0), I_(cs1), I_(cs2), and I_(cs3).

The modulation symbol d(0) is spread to a cyclically shifted sequencer(n,I_(cs)). When a one-dimensional spreading sequence corresponding toan (i+1)^(th) OFDM symbol in a slot is denoted by m(i), it can beexpressed as follows.

{m(0),m(1),m(2),m(3)}={d(0)r(n,I _(cs0)),d(0)r(n,I _(cs1)),d(0)r(n,I_(cs2)),d(0)r(n,I _(cs3))}

In order to increase UE capacity, the one-dimensional spreading sequencecan be spread by using an orthogonal sequence. An orthogonal sequencew_(i)(k) (where i is a sequence index, 0≦k≦K−1) based on a spread factorK is as follows.

TABLE 4 Index K = 4 K = 3 (i) [w_(i)(0), w_(i)(1), w_(i)(2), w_(i)(3)][w_(i)(0), w_(i)(1), w_(i)(2)] 0 [+1, +1, +1, +1] [+1, +1, +1] 1 [+1,−1, +1, −1] [+1, e^(j2π/3), e^(j4π/3)] 2 [+1, −1, −1, +1] [+1,e^(j4π/3), e^(j2π/3)]

A different spread factor can be used for each slot.

Therefore, when any orthogonal sequence index i is given,two-dimensional spreading sequences {s(0), s(1), s(2), s(3)} can beexpressed as follows.

{s(0),s(1),s(2),s(3)}={w _(i)(0)m(0),w _(i)(1)m(1),w _(i)(2)m(2),w_(i)(3)m(3)}

The two-dimensional spreading sequences {s(0), s(1), s(2), s(3)} aresubjected to inverse fast Fourier transform (IFFT) and thereafter aretransmitted in corresponding OFDM symbols. Accordingly, the ACK/NACKsignal is transmitted on a PUCCH.

An RS of the PUCCH format 1b is also transmitted by cyclically shiftingthe base sequence r(n) and then by spreading it by the use of anorthogonal sequence. When CS indices corresponding to three RS OFDMsymbols are denoted by I_(cs4), I_(cs5), and I_(cs6), three cyclicallyshifted sequences r(n,I_(cs4)), r(n,I_(cs5)), and r(n,I_(cs6)) can beobtained. The three cyclically shifted sequences are spread by the useof an orthogonal sequence w^(RS) _(i)(k) having a spreading factor K=3.

An orthogonal sequence index i, a CS index I_(cs), and a resource blockindex m are parameters required to construct the PUCCH, and are alsoresources used to identify the PUCCH (or UE). If the number of availablecyclic shifts is 12 and the number of available orthogonal sequenceindices is 3, PUCCHs for 36 UEs in total can be multiplexed to oneresource block.

In the 3GPP LTE, a resource index n⁽¹⁾ _(PUCCH) is defined in order forthe UE to obtain the three parameters for constructing the PUCCH. Theresource index n⁽¹⁾ _(PUCCH) is defined to n_(CCE)+N⁽¹⁾ _(PUCCH), wheren_(CCE) is a number indicating a first CCE used for transmission of acorresponding DCI (i.e., a DL resource allocation used to receive DLdata corresponding to an ACK/NACK signal), and N⁽¹⁾ _(PUCCH) is aparameter reported by a BS to the UE by using a higher-layer message.

To configure a resource index for the PUCCH, the UE uses a resourceallocation of a PDCCH. That is, a lowest CCE index (or an index of afirst CCE) used for transmission of the PDCCH is n_(CCE), and theresource index is determined as n⁽¹⁾ _(PUCCH)=n_(CCE)+N⁽¹⁾ _(PUCCH).

Time, frequency, and code resources used for transmission of theACK/NACK signal are referred to as ACK/NACK resources or PUCCHresources. As described above, an index of the ACK/NACK resourcerequired to transmit the ACK/NACK signal on the PUCCH (also referred toas an ACK/NACK resource index or a PUCCH index) can be expressed with atleast any one of an orthogonal sequence index i, a CS index I_(cs), aresource block index m, and a PUCCH index n⁽¹⁾ _(PUCCH) for obtainingthe three indices. The ACK/NACK resource may include at least any one ofan orthogonal sequence, a cyclic shift, a resource block, and acombination thereof.

FIG. 10 shows a structure of a PUCCH format 3 in a normal CP case.

One slot includes 7 OFDM symbols. l denotes an OFDM symbol number in theslot, and has a value in the range of 0 to 6. Two OFDM symbols with l=1,5 are used as RS OFDM symbols for a reference signal, and the remainingOFDM symbols are used as data OFDM symbols for a UCI signal.

A symbol sequence d={d(0), d(1), d(23)} is generated by performingquadrature phase-shift keying (QPSK) modulation on 48-bit encoded UCI(e.g., encoded ACK/NACK). d(n)(n=0,1, . . . ,23) is a complex-valuedmodulation symbol. The symbol sequence d can be regarded as a set ofmodulation symbols. The number of bits of the UCI or a modulation schemeis for exemplary purposes only, and thus the present invention is notlimited thereto.

One PUCCH uses one resource block (RB), and one subframe includes afirst slot and a second slot. A symbol sequence d={d(0), d(1), d(23)} isdivided into two sequences d1={d(0), . . . , d(11)} and d2={d(12), . . .,d(23)}, each having a length of 12. The first sequence d1 istransmitted in the first slot, and the second sequence d2 is transmittedin the second slot. In FIG. 3, the first sequence d1 is transmitted inthe first slot.

The symbol sequence is spread with an orthogonal sequence w_(i). Symbolsequences correspond to respective data OFDM symbols. An orthogonalsequence is used to identify a PUCCH (or wireless device) by spreadingthe symbol sequence across the data OFDM symbols.

An orthogonal sequence w_(i)(k) (i is a sequence index, 0≦k≦K−1) basedon a spreading factor K is as follows.

TABLE 5 Index K = 5 K = 4 (i) [w_(i)(0), w_(i)(1), w_(i)(2), w_(i)(3),w_(i)(3)] [w_(i)(0), w_(i)(1), w_(i)(2), w_(i)(2)] 0 [+1, +1, +1, +1][+1, +1, +1, +1] 1 [+1, e^(j2π/5), e^(j4π/5), e^(j6π/5), e^(j8π/5)] [+1,−1, +1, −1] 2 [+1, e^(j4π/5), e^(j8π/5), e^(j2π/5), e^(j6π/5)] [+1, +1,−1, −1] 3 [+1, e^(j6π/5), e^(j2π/5), e^(j8π/5), e^(j4π/5)] [+1, −1, −1,+1] 4 [+1, e^(j8π/5), e^(j6π/5), e^(j4π/5), e^(j2π/5)] —

In two RS OFDM symbols, an RS sequence used in UCI demodulation ismapped and transmitted.

In the PUCCH format 3, only the orthogonal sequence is used todistinguish wireless devices. To allocate the orthogonal sequence forthe PUCCH format 3, a resource index may be defined similarly to thePUCCH format 1. A BS allocates a resource index set for the PUCCH format3 in advance to the wireless device through an RRC message. In addition,a resource index to be used in the resource index set is directlyindicated by the DL grant.

Now, a PUCCH resource allocation in multiple cells is describedaccording to an embodiment of the present invention.

A coordinated multi-point (CoMP) is a technique in which a wirelessdevice receives a signal from several BSs or transmits the signal to theseveral BSs, and is generally used to increase a data throughput at aboundary of a serving BS.

First, the following terms are defined.

Macro base station (M-BS): A BS to which a serving cell of amacro-wireless device (M-WD) belongs. It is also called a serving BS.

Pico BS (P-BS): A BS having a coverage partially or entirely overlappingwith the M-BS. The M-WD also belongs to the coverage of the P-BS. Thecoverage of the P-BS is smaller than a coverage of the M-BS, but thepresent invention is not limited thereto. The P-BS may also be called invarious terms such as a femto-BS, a home eNB, a closed subscriber group(CSG) cell, a reception point (RP), a transmission point (TP), etc.

Physical cell identity (PCID): A physical identity for identifying acorresponding BS. It may be directly acquired by the wireless device byreceiving a synchronization signal (e.g., a primary synchronizationsignal (PSS) and a secondary synchronization signal (SSS)) of thecorresponding BS.

Virtual cell identity (VCID): An identity allocated from a currentserving BS. The M-WD does not need to acquire the PCID of the P-BS evenif it belongs to the coverage of the P-BS since the M-BS is the servingBS. In order for the M-WD to perform transmission to the P-BS, the cellidentity of the P-BS may be required. An identity to be used as the cellidentity of the P-BS is reported by the M-BS, and this is called theVCID.

FIG. 11 shows an example of a CoMP scenario.

An M-WD1 and an M-WD2 receive a service from an M-BS. The M-WD1 belongsto a coverage of a P-BS1, and the M-WD2 belongs to a coverage of aP-BS2. A PCID of the M-BS is a PCID0, a PCID of the P-BS1 is a PCID1,and a PCID of the P-BS2 is a PCID2. That is, this is a case where PCIDsof all BSs are different.

The M-WD1 receives an EPDCCH/PDSCH from the M-BS. In addition, the M-WD1transmits ACK/NACK corresponding to the PDSCH to the ‘P-BS1’ on a PUCCH.The M-WD2 receives the EPDCCH/PDSCH from the M-BS. In addition, theM-WD2 transmits ACK/NACK corresponding to the PDSCH to the ‘P-BS2’ onthe PUCCH.

Since a BS for transmitting the EPDCCH and a BS for receiving the PUCCHare different from each other, it may be difficult to directly use theexisting PDCCH-PUCCH resource linkage. This is because a PUCCH resourceacquired from the PDCCH can be used by another BS.

The PUCCH resource region linked to the EPDCCH may be separated from aPUCCH resource region linked to the PDCCH. In addition, instead of thePCID, a VCID may be used in the PUCCH resource region linked to theEPDCCH. The PUCCH resource region implies a set of PUCCH resources thatcan be allocated for a corresponding PUCCH.

FIG. 12 shows a PUCCH resource allocation according to an embodiment ofthe present invention.

A PUCCH resource region linked to a PDCCH is separated in all of anM-BS, a P-BS1, and a P-BS2. That is, a PUCCH resource region linked to aPDCCH for the M-BS and a PDCCH-PUCCH resource region for the P-BS1 andthe P-BS2 are different from each other. This is a case where a highinterference is expected between the M-BS and the P-BS1/P-BS2.

The PUCCH resource region linked to the EPDCCH is shared in all of theM-BS, the P-BS1, and the P-BS2. That is, the PUCCH resource regionlinked to the EPDCCH (this is called a shared PUCCH resource region) isidentical in all of the M-BS, the P-BS1, and the P-BS2. A VCID is usedto identify a shared PUCCH resource region and the PUCCH resource regionlinked to the PDCCH.

In the shared PUCCH resource region, instead of the PCID, the VCID maybe used to configure the PUCCH. For example, I_(cs) of Equation 6 is acyclic shift index indicating a CS amount. I_(cs) is defined by apseudo-random sequence. A pseudo-random sequence generator may beinitialized based on the VCID.

FIG. 13 shows a PUCCH resource allocation according to anotherembodiment of the present invention.

A PUCCH resource region linked to a PDCCH is identical in all of anM-BS, a P-BS1, and a P-BS2. A PUCCH resource region linked to a PDCCHfor the M-BS, a PDCCH-PUCCH resource region for the P-BS1, and aPDCCH-PUCCH resource region for the P-BS2 are different from oneanother. This a case where a low interference is expected among theM-BS, the P-BS1, and the P-BS2.

The PUCCH resource region linked to the EPDCCH is shared in all of theM-BS, the P-BS1, and the P-BS2. The PUCCH resource region linked to theEPDCCH is identical in all of the M-BS, the P-BS1, and the P-BS2. A VCIDis used to identify a shared PUCCH resource region and the PUCCHresource region linked to the PDCCH.

FIG. 14 is another example of a CoMP scenario.

In comparison with the example of FIG. 11, a PCID of an M-BS, a PCID ofa P-BS1, and a PCID of a P-BS2 are all equal to PCID0. Wireless devicesbelonging to the P-BS1 and the P-BS2 may be all scheduled by an EPDCCHbased on a VCID.

FIG. 15 shows a PUCCH resource allocation according to anotherembodiment of the present invention.

Since a PDCCH is used only by an M-BS, only a PUCCH resource regionlinked to a PDCCH for the M-BS is present.

The PUCCH resource region linked to the EPDCCH is shared in all of theM-BS, the P-BS1, and the P-BS2. That is, the PUCCH resource regionlinked to the EPDCCH is identical in all of the M-BS, the P-BS1, and theP-BS2. A VCID is used to identify a shared PUCCH resource region and thePUCCH resource region linked to the PDCCH.

If a BS for receiving an EPDCCH/PDSCH differs from a BS for transmittinga PUCCH in a CoMP environment, an orthogonality may be damaged betweenPUCCH resources which are multiplexed in the same UL subframe due to adifference of PUCCH transmission power. Therefore, a PUCCH resource forCoMP and a PUCCH resource for non-CoMP are separated.

FIG. 16 is a flowchart showing an ACK/NACK transmission method accordingto an embodiment of the present invention.

In step S910, a wireless device receives an EPDCCH configuration from afirst BS. The first BS may be an M-BS.

The EPDCCH configuration may include information regarding one or moreEPDCCH sets. The EPDDCH set may correspond to one search space in whichthe EPDCCH is monitored, and may include one or more PRB pairs (orPRBs). For example, the EPDDC set may include at least any one of thefollowing fields.

TABLE 6 Field Content Identity Identity of EPDCCH set (or also referredto as an EPDCCH set index) Transport type To indicate distributedtransmission or local transmission RB allocation PRB pair for EPDCCH setPUCCH offset Offset for PUCCH resource RS scramble identity Scramblingsequence initialization parameter of DM RS for EPDCCH

Together with or independently from the EPDCCH configuration, theinformation regarding the VCID may be transmitted from the first BS tothe wireless device.

In step S920, the wireless device monitors a configured EPDCCH set, andreceives a DL grant on an EPDCCH.

In step S930, the wireless device receives a DL transport block on aPDSCH indicated by the DL grant.

In step S940, the wireless device transmits ACK/NACK for the DLtransport block to a second BS on a PUCCH on the basis of a PUCCHresource. The second BS may be a P-BS. The PUCCH may be configured onthe basis of a PUCCH resource and a VCID.

A PUCCH resource for a PDSCH scheduled by the EPDCCH may be determinedas follows.

In a first embodiment, an additional offset may be defined in a resourceindex for a PUCCH.

For example, a resource index n⁽¹⁾ _(PUCCH) is defined as follows.

n _(PUCCH) ⁽¹⁾ =n _(ECCE,q) +N _(PUCCH,q) ⁽¹⁾ +No  [Equation 7]

Herein, n_(ECCE,q) is a number indicating a first ECCE in which acorresponding EPDCCH is detected in an EPDCCH set q, N⁽¹⁾ _(PUCCH) is avalue reported by a higher layer for the EPDCCH set q, No is an offsetadded due to CoMP. These values may be reported on a higher layermessage or on a corresponding EPDCCH.

In a second embodiment, information regarding a position (e.g., a startpoint) of a PUCCH resource region for an EPDDCH may be reported by a BS.The information may be cell specific or a wireless device specific orwireless device group specific.

In a third embodiment, a DL grant on an EPDCCH may include PUCCHresource allocation information.

For example, the DL grant may include information regarding a referencevalue used to determine a PUCCH resource.

For another example, the BS reports information regarding a plurality ofPUCCH resource candidates in advance to the wireless device through anRRC message, etc. In addition, the DL grant may include an indicatorregarding a PUCCH resource to be used among the plurality of PUCCHresource candidates.

For example, if the indicator is 2 bits, it may be expressed as follows.

TABLE 7 Bit Contents ‘00’ 1^(st) PUCCH resource ‘01’ 2^(nd) PUCCHresource ‘10’ 3^(rd) PUCCH resource ‘11’ 4^(th) PUCCH resource

TABLE 8 Bit Contents ‘00’ Use of existing PDCCH-PUCCH linkage ‘01’1^(st) PUCCH resource ‘10’ 2^(nd) PUCCH resource ‘11’ 3^(rd) PUCCHresource

The reference value may be determined by the RRC message, and theindicator may indicate an offset from the reference value.

TABLE 9 Bit Contents ‘00’ Offset 0 ‘01’ Offset −1 ‘10’ Offset −2 ‘11’Offset −3

In Table 7 to Table 8 above, the number of bits of the indicator and thecontent of a corresponding value are for exemplary purposes only.

In a fourth embodiment, the wireless device may determine a PUCCHresource region for an EPDCCH according to a size of a control regionfor a PDCCH. For example, a resource index n⁽¹⁾ _(PUCCH) may be definedas follows.

n _(PUCCH) ⁽¹⁾ =n _(ECCE,q) +N _(PUCCH,q) ⁽¹⁾ +Fo  [Equation 8]

Herein, Fo is a value acquired based on a size of a control region.

A PUCCH resource region may be determined according to a DCI format, asearch space type, an aggregation level, a wireless device-specificparameter, etc.

FIG. 17 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

A BS 50 includes a processor 51, a memory 52, and a radio frequency (RF)unit 53. The memory 52 is coupled to the processor 51, and stores avariety of information for driving the processor 51. The RF unit 53 iscoupled to the processor 51, and transmits and/or receives a radiosignal. The processor 51 implements the proposed functions, procedures,and/or methods. In the aforementioned embodiment, an operation of the BSmay be implemented by the processor 51. The processor 51 may configurean EPDCCH, and may transmit the EPDCCH and/or a PDCCH. The processor 51may support an HARQ operation, and may receive HARQ ACK/NACK.

A wireless device 60 includes a processor 61, a memory 62, and an RFunit 63. The memory 62 is coupled to the processor 61, and stores avariety of information for driving the processor 61. The RF unit 63 iscoupled to the processor 61, and transmits and/or receives a radiosignal. The processor 61 implements the proposed functions, procedures,and/or methods. In the aforementioned embodiment, an operation of thewireless device may be implemented by the processor 60. The processor 61may monitor an EPDDCH, and may transmit HARQ ACK/NACK.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The RF unit may include a base-bandcircuit for processing a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory and may be performed by the processor. The memory may be locatedinside or outside the processor, and may be coupled to the processor byusing various well-known means.

Although the aforementioned exemplary system has been described on thebasis of a flowchart in which steps or blocks are listed in sequence,the steps of the present invention are not limited to a certain order.Therefore, a certain step may be performed in a different step or in adifferent order or concurrently with respect to that described above.Further, it will be understood by those ordinary skilled in the art thatthe steps of the flowcharts are not exclusive. Rather, another step maybe included therein or one or more steps may be deleted within the scopeof the present invention.

What is claimed is:
 1. A method for transmitting hybrid automatic repeatrequest (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement(NACK) in a wireless communication system, the method comprising:receiving, by a wireless device, downlink control information from afirst base station on a downlink control channel; receiving, by thewireless device, a downlink transport block from the first base stationon a downlink shared channel according to the downlink controlinformation; and transmitting, by the wireless device, ACK/NACK for thedownlink transport block to a second base station on an uplink controlchannel, wherein information regarding a cell identity for the uplinkcontrol channel for the second base station is received from the firstbase station, and wherein the downlink control information includes anindicator used to determine a radio resource for the uplink controlchannel.
 2. The method of claim 1, further comprising: receiving, by thewireless device, information regarding a plurality of uplink channelresource candidates from the first base station, wherein the indicatorindicates one of the plurality of uplink channel resource candidates. 3.The method of claim 1, wherein the indicator indicates an offset of aresource index used to determine a radio resource for the uplink controlchannel.
 4. The method of claim 1, wherein the transmitting of theACK/NACK on the uplink control channel comprises: modulating theACK/NACK to generate a modulation symbol; spreading the modulationsymbol to a sequence which is cyclically shifted by a cyclic shiftamount; and transmitting the spread sequence.
 5. The method of claim 4,wherein the cyclic shift amount is determined based on the cellidentity.
 6. The method of claim 4, wherein the modulation symbol ismodulated by using binary phase shift keying (BPSK) modulation orquadrature phase shift keying (QPSK) modulation.
 7. The method of claim1, wherein a search space in which the downlink control channel isdetected is defined by one or more physical resource block (PRB) pairs.8. The method of claim 7, wherein orthogonal frequency divisionmultiplexing (OFDM) symbols in which the downlink control channel isreceived partially or entirely overlaps with OFDM symbols in which thedownlink shared channel is received.
 9. A wireless device fortransmitting hybrid automatic repeat request (HARQ)positive-acknowledgement (ACK)/negative-acknowledgement (NACK) in awireless communication system, the wireless device comprising: a radiofrequency (RF) unit configured to transmit and receive a radio signal;and a processor operatively coupled to the RF unit and configured to:receive downlink control information from a first base station on adownlink control channel; receive a downlink transport block from thefirst base station on a downlink shared channel according to thedownlink control information; and transmit ACK/NACK for the downlinktransport block to a second base station on an uplink control channel,wherein information regarding a cell identity for the uplink controlchannel for the second base station is received from the first basestation, and wherein the downlink control information includes anindicator used to determine a radio resource for the uplink controlchannel.
 10. The wireless device of claim 9, wherein the processorreceives information regarding a plurality of uplink channel resourcecandidates from the first base station, and wherein the indicatorindicates one of the plurality of uplink channel resource candidates.11. The wireless device of claim 9, wherein the processor is configuredto transmit the ACK/NACK on the uplink control channel by: modulatingthe ACK/NACK to generate a modulation symbol; spreading the modulationsymbol to a sequence which is cyclically shifted by a cyclic shiftamount; and transmitting the spread sequence.
 12. The wireless device ofclaim 11, wherein the cyclic shift amount is determined based on thecell identity.